The present invention relates to the use of sulfated lime (mainly calcium sulfate) derived from dry desulfurization of flue gas as a polymer filler.
It is known that flue gas produced by fuels containing sulfur compounds must be purified to remove the sulfur oxides it contains before it can be released into the atmosphere.
Different methods have been proposed in the prior art to achieve this goal. The desulfurization processes currently in use include those comprising contacting the flue gas with particles of an absorbing material containing at least one basic compound of an alkaline earth metal (see, for example, French Patent [FR] A-2 636 720). The most frequently used absorbents are, in particular, limestones (CaCO.sub.3), slaked lime (Ca(OH).sub.2) and the dolomites (CaMg(CO.sub.3).sub.2). The absorbents usually have a particle size of 1 to 100 microns.
The flue gas desulfurization is carried out at an elevated temperature above 700.degree. C. and, at this temperature, the above-said compounds are at least partly converted into quicklime with elimination of water and carbon dioxide.
Upon contact with flue gas, the quicklime particles react with the sulfur oxides present therein with formation of calcium sulfate and, thereafter, they contain a large amount of this compound.
In the present description and the attached claims, said particles containing a large amount of calcium sulfate, typically derived from the desulfurization of flue gas, shall be referred to by the term "sulfated lime".
Sulfated lime usually contains in weight percent 40 to 80% of calcium sulfate, CaSO.sub.4, and 10 to 50% of quicklime, CaO, the remainder consisting essentially of 0 to 10% of calcium carbonate, CaCO.sub.3, and 0 to 15% of slaked lime, Ca(OH).sub.2. These, of course, are average compositions, it being possible to use within the scope of the invention sulfated lime of a different composition.
Sulfated limes usually have a bulk density of 0.35 to 0.80 g/cm.sup.3. The particle size is usually between 1.times.10.sup.-6 and 100.times.10.sup.-6 m, with an average of 2.times.10.sup.-6 to 40.times.10.sup.-6 m.
The large-scale use of such absorbents for the desulfurization of flue gas can produce annually about ten thousand metric tons of sulfated lime per desulfurization unit. Said sulfated lime is thus an available material.
Applicant and others have for a long time been studying the possible use of these desulfurization by-products and unexpectedly has now found that the sulfated lime derived from the desulfurization of flue gas can advantageously replace the fillers incorporated into polymers.
It is known to incorporate different powdered fillers into polymers such as polyvinyl chloride and, more particularly, into elastomers such as natural or synthetic rubbers. Usually one distinguishes between inert fillers, namely those that are used simply to reduce the amount of polymer in a formulation by incorporating into said polymer a low-cost material, for example chalk, and reinforcing or semi-reinforcing fillers capable of modifying the properties of the polymer. The latter kinds of fillers include carbon black, precipitated silica and kaolin which when incorporated, for example, into elastomers enables said elastomers to meet certain industrial specifications.